Diagnostic Technique of Abnormalities in Ball Bearings with an Ultrasonic Method

The sensitive and quantitative measurement method will be important to evaluate the lubrication condition and to predict the remaining bearing life-time. Observations of the size and number of damage or wear particle, and the fluctuation of the ball load are useful for the quantitatively evaluation of operation abnormalities in a ball bearing. The vibration and AE (Acoustic Emission) method are usually used as the effective diagnostic technique. However, it is difficult to detect those items individually by these methods. Then, the ultrasonic technique was attempted to use as a semi-quantitative measuring method for sensing these quantities. Ultrasonic wave pulses are entered into a bearing housing from an ultrasonic probe attached to the outside of the housing. Those pulses are partially reflected from the interface between the housing and the outer ring of the bearing. Because of the different acoustic impedance between the solid-to-solid contact and the contact with an intervening gas or liquid layer, the height of the reflected echo depends on the real contact area or on the local contact pressure at the interface. In a normal condition, the height of the filtered echo pulses produces smooth, nearly sinusoidal peaks corresponding to passages of the balls. Semi-quantitative evaluation of abnormality becomes possible on the basis of the observation of magnitude and frequency of the echo height fluctuation. Diagnosis of bearing abnormality was performed as a standard of evaluation in size, fluctuation velocity and mode of echo height fluctuation. Then algorithms for the judgment of those abnormalities were added in diagnostic software, and abnormal width or fluctuation rate of ball support load were evaluated semiautomatically. As a result, the measurement of model indentation with 0.3mm width added to inner ring and the quantitative evaluation of growth of an indentation were enabled.